The precise metering of materials (e.g., dry solids) is important in many applications, including numerous manufacturing processes in various industries. Usually when material is continuously metered into a process, it must be precisely controlled at a specific feed rate so that the process functions as designed, the product formulation is correct, and the quality of the end product of the process does not suffer. In other applications, it is important to just keep track of the amount of material that has passed through a process, and controlling the feed rate is less important, or not important at all. Many of these applications are automated, and productivity concerns demand that they proceed without human intervention to the greatest extent possible.
Various kinds of weigh feeders have been used for weighing and feeding materials such as sand, gravel, grain, foodstuffs, chemicals, pharmaceuticals, ceramics, etc. In general, material is provided to a weigh feeder continuously or periodically and the weigh feeder discharges the material at a desired output rate. Different weigh feeders, however, have different capabilities, which depend on the design of the weigh feeder and its principle of operation. Weight-loss and weigh belt feeders are two types of commonly used weigh feeders.
Weigh belt feeders can weigh material as the material is transported by a moving belt and usually receive a continuous supply of material, generally from an overhead storage supply. In one configuration (e.g., the Acrison, Inc., 260 Belt Weigher/Feeder), material travels from a storage supply, down a chute and onto a rear portion of the belt, which is not weighed. As the belt moves, the material on the belt passes over a weighing section, and a weight signal is produced that corresponds to the weight of material traveling across the weighing section. The weight signal is processed in conjunction with another signal, representing the speed of the belt, by the weigh feeder's controller to derive a feed rate signal. The feed rate signal is compared to the feed rate selected by the user, and the weigh feeder's controller continuously adjusts a variable speed drive powering the belt to maintain the desired feed rate.
A weigh belt feeder may also utilize a feeding mechanism to actively feed material onto the belt (e.g., a screw conveyor/feeder, another belt, a vibratory tray device, etc.). Although such active feeding (or prefeeding) is different from the method of gravimetric feeding described above, the material on the belt is weighed in a similar manner. Such active feeding of material onto the weigh belt can provide a greater degree of physical control over the material being fed. In this mode of operation, the weigh belt moves at a fixed constant speed, and the feed rate of the feeding mechanism is variable. Thus, the weigh feeder's controller continuously modulates the output of the feeding mechanism that feeds material onto the belt to maintain a selected feed rate of material discharging off the belt. Material is usually provided to the feeding mechanism directly from a storage supply, for example, a hopper or silo.
A different type of weigh belt feeder (e.g., the Acrison, Inc., 203/210 weigh belt feeders) operates by weighing the entire weigh belt assembly, while a pre-feeder (e.g., a screw conveyor and/or feeder, another belt, or a vibratory type device) meters material onto the weigh belt, which operates at a fixed constant speed. The output of the pre-feeder, which is equipped with a variable speed drive, is continuously modulated by the weigh feeder's controller so that the rate at which material passes across the weigh belt accurately matches the selected feed rate. In such a weigh feeder, material is also usually supplied to the pre-feeder directly from a storage supply.
A weight-loss feeder (e.g., the Acrison, Inc. 400 Series weight-loss feeders) comprises a material supply hopper and a feeding mechanism mounted on a scale. As material is fed out of the scale-mounted metering/supply system, a decreasing weight signal is produced, which is processed by the weigh feeder's controller in conjunction with a time signal to calculate a feed rate. The feeding mechanism of a weight-loss weigh feeder is equipped with a variable speed drive so that its feed rate output can be continuously modulated by the weigh feeder's controller in order to maintain the selected feed rate. The supply hopper of a weight-loss weigh feeder can be periodically refilled.
In other processes, it is desirable to know how much material has been fed, and control of the amount is not necessary. These applications can utilize a weigh belt weigher, which only weighs the amount of material being conveyed by the belt over a given amount of time. Like the two types of weigh belt feeders discussed above, weigh belt weighers convey material via a moving belt. Indeed, many devices employed in industry convey material via a moving belt.